Have you tried to add PLF repository? If not, you can visit this site:
http://easyurpmi.zarb.org/.
Click 'Add PLF medias' from there. I guess the browser will ask then how to open a file named 'easyurpmi.urpmi-media', you just accept the default. Then it will ask a lot of confirmations (openSUSE does the same with One Click Install) and finally the repository will be added.
Then it would be possible to install MP3-related packages through Mandriva's graphical package manager. AFAIK you have to install
gstreamer0.10-plugins-ugly for mp3's to play in Rhythmbox.
(Disclaimer: I do not use Mandriva now, so it's just what I remember from my past experiences and what I guess from what Google told me )